Technical Field
The present invention relates to photovoltaic devices, and more particularly to exfoliation methods and devices for exfoliating absorber layers comprised of a chalcogenide compound, such as, Cu—Zn—Sn—S/Se (CZTSSe).
Description of the Related Art
Photovoltaic (PV) devices are typically fabricated from polycrystalline Si. Since Si is not a particularly good light absorber in the wavelength range emitted by the sun, it must necessarily be made thick, typically 300 micrometers or more. This leads to relatively heavy solar panels fabricated with polycrystalline Si and added expense. The expense and weight has driven the search for efficient thin film PV materials. Thin film PV absorber materials (e.g., CuInGaSe,S or CIGS, CdTe) may be about 150-300 times thinner, with thicknesses ranging from 0.5-3 microns. Such a thin absorber is made possible by the appropriate choice of materials that are extremely absorptive in the wavelength regime of peak solar irradiance.
Optimizing the performance of thin film absorbers while reducing their toxicity and dependence on rare and expensive elements suggests the use of earth abundant materials such as CuZnSnSe,S (CZTS) for example. In all cases, there is a need to both increase power conversion efficiency and reduce costs associated with manufacturing. A thin film PV device based on absorbers such as CIGS or CZTS involves the deposition of precursor elements or compounds typically onto Mo/soda lime glass. Precursors can be deposited by vacuum vapor deposition of elements onto this substrate or by solution deposition of precursor inks in a nitrogen glove box.
Upon deposition of the absorber precursors, a high temperature anneal step needs to be carried out to coarsen the grain size of a resultant polycrystalline material. The absorber material is a p-type semiconductor and to form a p-n junction, an n-type “buffer” material needs to be deposited, followed by additional n-type layers such as ZnO and indium tin oxide or Al doped ZnO and finally metal lines for current collection. The n-type buffer and subsequent layers are deposited at low or room temperature as their electrical properties are compromised by heating to temperatures above 200 C. Devices fabricated in this manner are called “substrate” grown devices. A second type of device in which the n-type materials including the buffer are deposited first followed by the absorber material and back contact are called “superstrate” grown devices. An example of this device fabrication is a CdTe based PV device. Typically, though, most thin film PV devices are grown in the substrate mode because the high temperature anneal needed to fully form the large grain polycrystalline absorber would otherwise destroy the buffer if grown in the superstrate mode. More specifically, if CIGS or CZTS is deposited on CdS or similar buffer materials, then annealed at temperatures above 300 C, interdiffusion of the buffer and absorber materials renders the device inoperable.
One of the approaches to increasing power conversion efficiency is to modify the back contact of a photovoltaic device. Access to the back contact of a typical PV device is limited because either the device is grown in a “substrate” mode, or the fully formed device needs to be separated from the Mo/glass substrate to provide access to the back contact region of the PV absorber. Separating the device from the Mo/glass substrate often results in damage to the device.
Typical separation processes involve introducing substantial thermal or mechanical stress to release the absorber from the Mo back contact. Both thermal and mechanical stressing and separation only marginally work due to cracking and pinhole formation as a result of the trauma of the separation process. In addition, these processes can only be carried out on small devices.